Production and secretion of progesterone in vitro and presence of platelet activating factor (PAF) in early pregnancy of the marsupial, Macropus eugenii.

The corpus luteum (CL) of the tammar is suppressed by lactation. Removal of the sucking pouch young induces reactivation of the CL which produces a peak in plasma progesterone concentration on Day 5, 6 or 7; reactivation of the embryo after diapause precedes by one day this transient peak of progesterone. This study examines progesterone production and secretion in vitro by the CL and the production of platelet activating factor (PAF) by the endometrium during this stage of pregnancy (Days 0 and 3-8). Blood samples were collected twice daily to determine for each animal (n = 28) at autopsy the relationship of PAF production to the day of the progesterone peak. Significant changes in basal plasma progesterone concentrations were used to define four groups according to the time the animals were killed: Group A, Day 0; Group B, other animals showing no significant change; Group C, peripheral progesterone concentration increasing; Group D, peripheral progesterone concentration decreasing after the peak. At autopsy on successive days, blood samples were taken from the heart, caudal vein and uterine branch of the ovarian vein, and CL and endometria were prepared for explant culture. Progesterone contents of pre- and post-incubation luteal tissues and of medium were determined at 4, 12 and 24 h to estimate production and secretion rates. The values for all progesterone parameters from animals in Group C were significantly higher than in the other groups. It is concluded that the progesterone peak results from a change in rate of production and secretion of progesterone by the CL, one day before the peak in peripheral plasma, and that this change could provide the first signal to the uterus. Endometrium was incubated for 24 h and the medium assayed for PAF. High concentrations of PAF were detected in half the animals in Groups B and C, that is, before or at the time progesterone was increasing. This is the first measurement of PAF in any marsupial and the first demonstration of the release of PAF by the endometrium. The detection of PAF provides a candidate for an endometrial signal that is responsive to the rising progesterone concentration in the utero-ovarian vein draining the CL-bearing ovary and that may stimulate reactivation of the blastocyst.